The present invention relates to casting techniques and apparatus. More particularly, the invention is directed towards techniques and apparatus for forming anodes, preferably from lead alloys, as well as anodes formed thereby.
The techniques described below in connection with the present invention are particularly contemplated in connection with relatively large anodes of a type employed as an insoluble anode in the electrowinning of copper. Accordingly, novel features of the present invention are described below in connection with such an electrowinning process. However, it will be apparent that the present invention may also be employed for other purposes, for example, in the casting of non-ferrous materials for use as insoluble anodes as well as in other applications.
The Electrowinning Process: PA0 Insoluble Anodes Used In Electrowinning: PA0 Important Characteristics For Insoluble Anodes:
A brief description of the electrowinning process for the recovery of copper is set forth below to permit a better understanding of the present invention. Generally, a solution of concentrated copper in sulfuric acid is formed, usually by leaching of copper ore. An acid concentration in the range of 100-200 gms. per liter of sulfuric acid is generally necessary in order to place sufficient copper in solution. A corresponding copper concentration may be in the range of approximately 30-50 gms. per liter for a pregnant electrolyte to be introduced into the electrowinning process.
The copper-laden solution is then introduced into one or more electrolytic cells each containing a series of anodes and cathodes.
Within the electrolytic cells, the anodes are substantially insoluble with an electrical potential impressed between the anodes and cathodes tending to cause migration of copper from the electrolyte toward the cathode with metallic copper being deposited or built up on each of the cathodes. Fresh electrolyte is constantly supplied to the cells. Sulfuric acid solution is recycled from the cells for the leaching or solution of additional copper which is then again introduced into the electrowinning cells.
The cathodes containing the build-up of metallic copper are periodically removed from the cells and replaced by fresh electrodes or starter sheets to permit continued deposition. The cathodes removed from the cells contain relatively pure copper, for example in the range of 99% purity. A portion of the copper resulting from the electrowinning process is accordingly used directly in copper consuming applications. However, since many applications require copper of even higher purity, it is also common to further refine copper obtained from the electrowinning process by conventional electro-refining techniques.
The theoretically insoluble anodes are a particular source of concern within the electrowinning process and have been the object of substantial development efforts throughout the relatively long history of the electrowinning process. Problems arising in connection with the anodes tend to develop because of the infeasibility of providing a completely insoluble anode which is still capable of adequate electrical performance within the cell. It has been commonly found that material from the anode tends to become dissolved in small quantities within the electrolyte with a portion of the dissolved anode material being collected or trapped upon the cathode together with metallic copper.
For some time, insoluble anodes used in the electrowinning of copper have been formed from lead or lead alloys. Relatively limited amounts of lead have been found within the copper deposit on the cathodes. However, in many copper consuming applications, the acceptable limits for lead as an impurity are very low, commonly in the range of 10-20 parts per million. Much of the effort directed toward development of improved anodes has therefor concerned techniques for making lead or lead alloy anodes having high hardness and resistance to corrosion or exfoliation while also maintaining adequate mechanical and dimensional stability in the anodes to permit their continued use in electrowinning cells over substantial periods of time.
In the recent past, the most common lead alloy employed in electrowinning anodes as one containing substantial quantities of antimony, for example, 5-15% Sb by weight. In addition to such binary alloys, ternary and quaternary alloys including lead and antimony have also been commonly employed with the additional alloying agents being selected from a broad group including arsenic, bismuth, tin, cadmium, thallium, tellurium, mercury, colbalt, barium, strontium, selenium, tantalum, smooth platinum, etc. More recently, lead-silver alloys have been investigated, particularly those in ternary form including a third alloy such as arsenic or bismuth.
Generally, anodes formed from lead alone do not have sufficient hardness or resistance to corrosion or exfoliation to permit their use in electrowinning because of excessive lead migration with the lead being deposited or entrapped upon the cathodes together with metallic copper. The various alloying agents tend to increase hardness and corrosion resistance while also contributing to mechanical and dimensional stability, all of these being particularly desirable characteristics for insoluble anodes in the electrowinning process.
Calcium is an additional material of particular interest within such lead alloys. Although calcium may be employed within binary, ternary or even quaternary alloys in widely varying amounts, the most useful concentrations for calcium in such alloys are believed to be within the range of 0.01 to 0.1% by weight.
The attractive corrosion resistance and superior mechanical properties of lead alloys which contain calcium have been known for many years as is demonstrated to some degree by the use of calcium alloys in the manufacture of battery grids. However, it is to be particularly noted that manufacturing techniques and operating performance requirements for battery grids are different from the requirements for insoluble anodes such as are employed in the electrowinning process. Experience in the battery field may be taken to reinforce the conclusion that lead-calcium alloys are more difficult to cast or otherwise form into a usable configuration.
It is noted in passing at this point that the present invention provides one or more casting techniques which, either alone or in combination, permit the casting of a lead-calcium anode having greatly superior properties of corrosion resistance and mechanical and dimensional stability, particularly for use in electrowinning processes. However, it is again emphasized that the casting techniques of the present invention are not limited merely to lead-calcium alloys employed in insoluble anodes for use in the copper electrowinning process. On the other hand, because of the particular effectiveness of the present invention for forming such anodes, the preferred embodiment and examples of the present invention, as described below, are directed in large part toward such a combination.
It is also noted at this point that the techniques and apparatus provided by the present invention are also specifically applicable to more complex lead-calcium alloys, for example ternary alloys which include silver or tin, for example, as well as calcium.
It was indicated above that the purpose of employing lead alloys is to improve resistance to corrosion or exfoliation as well as to enhance both dimensional and mechanical stability. These requirements are relatively complex for insoluble anodes of the type employed within the electrowinning process. To provide additional background in this connection, it is noted that the "insoluble" or "inwer" anodes are immersed in sulfuric acid solution contained by electrowinning cells. Within the electrolytic process or under generally similar conditions commonly employed to stabilize or precondition the anode surface, lead within the anode tends to react with the sulfuric acid and also with air bubbles generated by electrolysis upon the surface of the anode. Interaction of these materials under electrolytic or similar stabilizing conditions tends to cause formation of a film upon the lead or lead alloy anode. The film principally consists of lead dioxide (PbO.sub.2) which acts as a semi-conductor, thus enabling electrical conductance through the anode and particularly enhancing its corrosion resistance. Lead oxide (PbO) and lead sulfate (PbSO.sub.4) are also present during various stages of the film formation and are characterized as being generally poor conductors.
It is theorized that the phenomenon of initial film formation and subsequent film reformation is important in maintaining the corrosion resistance of the anode. In any event, it has been found that the chemical and physical characteristics of the anode are important factors affecting formation of the abovenoted film and accordingly are important in achieving maximum corrosion resistance of the anode.
In addition, certain chemical and physical characteristics of the anode are also important in determining its mechanical and dimensional stability as was suggested above. These chemical and physical properties of the anode, which may affect corrosion resistance and/or mechanical and dimensional stability are summarized below.
Initially, chemical composition of the anode is of critical importance as suggested by the preceding discussion of the various lead alloys which have been developed. For example, it was clearly indicated above that certain lead alloys, particularly those including calcium as well as other binary, ternary and quaternary alloys, contribute both to the corrosion resistance of the anode as well as its mechanical strength and dimensional stability. It is also believed important to maintain in uncombined form the basic lead component except for the presence of precipitates formed with and between the various alloying agents. For example, in a lead-calcium alloy, a precipitate of lead calcium (PbCa.sub.3) is believed to be an important factor contributing to the improved characteristics of anodes formed from such alloys.
In respect to one aspect of the present invention, it is theorized that other compound formations of lead, in particular, within the anode, may be undesirable. In this regard, it is particularly believed that the combination of lead with oxygen to form either lead oxide or lead dioxide within the anode may undesirably interfere with subsequent formation of a stabilizing or conditioned film, as noted above.
Uniform precipitate distribution is an additional desirable characteristic to be considered in connection with alloy composition of the type discussed immediately above. Particularly with alloy compositions such as lead-calcium, it is believed that precipitate formation and uniform distribution of the precipitate throughout a matrix of lead contributes particularly to corrosion resistance and the related characteristic of surface hardness.
The characteristics of high density and low porosity are believed to be interconnected and jointly contribute again to the characteristics of corrosion resistance and surface hardness for the anode. These characteristics may also affect in part mechanical and dimensional stability of the anode.
Grain size is another characteristic which is believed to provide an important contribution to both corrosion resistance and mechanical characteristics such as hardness. Generally, it is believed that for a non-ferrous metal such as lead, it is desirable to maintain a relatively large or coarse grain size. Here again, the characteristic of coarse or large grain size is particularly important for the lead matrix in an alloy such as lead-calcium. It is assumed that relatively large or coarse grain size in such an alloy contributes both to film formation, as discussed above, and possibly also to uniform precipitate distribution. This supposition again illustrates that the many characteristics discussed herein are interrelated or interdependent upon each other.
Finally, it is believed desirable to form a surface upon the anode which may be characterized as uniform, continuous or generally smooth. It is believed that the nature of the initial surface formed upon the anode is of substantial importance. It is again theorized that the surface characteristics contribute importantly to proper film formation, as discussed above. Possibly, the combined characteristics of a smooth surface and relatively large grain size tend to promote development of a uniform film upon an anode which importantly minimized corrosion within the electrolytic bath. It will be noted below that in conjunction with the present invention, anodes formed according to the procedure described below tend to have the appearance of being "rolled" or "galvanized". In any event, the surface characteristics of the anode formed according to the present invention are believed to contribute importantly to its value within an electrowinning process.
In summary, the preceding comments have been directed toward a discussion of basic anode characteristics including good resistance to corrosion or exfoliation, mechanical stability or "strength", dimensional stability and relative hardness, both on the anode surface and within the anode interior. In this connection, it is important to note that as material is eventually lost from the anode, those portions originally within the anode interior then form its surface.
In addition, the preceding discussion emphasized the characteristics of chemical composition, uniform precipitate distribution, high density and low porosity, large or coarse grain size znd initial surface characteristics of the anode.
This discussion of anode characteristics is set forth above in some detail in order to emphasize advantages of the present invention. With the exception of chemical composition, the other basic anode characteristics discussed above are believed to be primarily dependent upon the method of casting or forming the anode, either alone or in conjunction with the chemical composition.